1. Field of the Invention
The present invention relates to an optical pickup device and an optical element used for the optical pickup device.
2. Related Art
Recently, in an optical pickup device, the shortening of wavelength of a laser light source used as a light source for reproducing the information recorded on an optical disk or recording information on an optical disk has progressed. For example, a laser light source having a wavelength of 405 nm such as a blue-violet laser diode and a blue-violet SHG laser performing the wavelength conversion of an infrared laser diode using a second-harmonic generation is being put to practical use.
When these blue-violet laser light sources are used, 15 to 20 GB of information can be recorded on an optical disk having a diameter of 12 cm in case of using an objective having the same numerical aperture (NA) as that of a digital versatile disk (hereinafter simply referred to as a DVD). In the case where the NA of an objective is raised up to 0.85, it becomes possible to record the information of 23 GB to 27 GB on an optical disk having a diameter of 12 cm. Hereinafter, in the present specification, an optical disk and a magneto-optical disk which use a blue-violet laser light source are generically called as a “high density optical disk”.
Now, two standards have been proposed as those of a high density optical disk until now. One is Blu-ray Disc (hereinafter simply referred to as BD) using an objective having an NA of 0.85 and having a protective layer of 0.1 mm in thickness, and the other is HD DVD (hereinafter simply referred to as HD) using an objective having an NA of 0.65 to 0.67 and having a protective layer of 0.6 mm in thickness. In consideration of the possibility of the distribution of high density optical disks in conformity with these two standards in the market in the future, an optical pickup device for compatibility with which also the recording and the reproducing of an existing DVD can be performed is important for both of the high density optical disks, and especially one-lens system which implements the compatibility with an objective is the most ideal form.
As for a compatible optical pickup device, as a correction method of spherical aberration caused by the differences in wavelengths of light beams to be used for a plurality of optical disks and in the thicknesses of protective substrates, a technique of changing the degree of the divergence of an incident light beam to an objective optical system, or of providing a diffractive structure on the optical surface of an optical element constituting an optical pickup device has been hitherto known.
Moreover, BD has a problem in which a chromatic aberration is produced when an instantaneous wavelength variation is generated since the focal depth of an objective is small owing to the smallness of the area of a spot converged on the information recording surface of BD and the resulting extreme height of the NA thereof in comparison the other recording media. Accordingly, a technique of making the characteristic of a lens to the variations of wavelengths (wavelength characteristic) by means of a diffractive structure has been known.
Moreover, an optical pickup device is configured by combining various optical elements such as an objective, a coupling lens and a beam expander, and these optical elements are frequently made of a plastic, which is light in weight and inexpensive in price. However, since a plastic has a feature in which the refractive index changes with temperature changes, for example, an objective made of a plastic has a problem in which a spherical aberration is generated in the over direction by a temperature rise.
Accordingly, a technique for offsetting the spherical aberration generated in the over direction owing to a temperature rise by providing a diffractive structure on the optical surface of an objective to generate the spherical aberration in an under direction by the diffractive structure in order to improve the characteristic of a lens to such a temperature change (temperature characteristic) is known.
As described above, the diffractive structure has been used for various uses such as compatibility achievement and the improvement of a wavelength characteristic and a temperature characteristic recently, and consequently a problem of the difficulty of a lens design which gives such a function only to the diffractive structure provided in an objective for example is produced. Accordingly, a technique for achieving the compatibility between two types of optical disks by providing a diffractive structure to an objective and also by providing a diffractive structure to a collimate lens formed separately form the objective to correct the chromatic aberration is known (see JP-Tokukai-2001-60336A).
However, even if the technique described in the JP-Tokukai-2001-60336A is used, the improvement of the above-mentioned temperature characteristic and the wavelength characteristic cannot be said to be sufficient, and technical means capable of improving the degree of freedom of a lens design is desired.